Such pressure limiting valves are known for hydraulic mine props, which valves in operative position have a working space provided with an inlet opening connected with the high-pressure space and outlet opening connected with the low-pressure space of the mine prop, the working space has a sealed movable part with a valve seat, as well as a cover plate fitted to the valve seat in a closed condition, and a spring that presses the movable part against the cover plate and a spring sustaining the cover plate, suitably a gas spring.
In these known valves, the valve seat rests on the cover plate made of elastic synthetic material. The valve seat forms an annular orifice, through which the actuating liquid of higher pressure than permissible flows from the high-pressure space of the hydraulic prop into the low-pressure space, when the valve seat--under the influence of the opening force exceeding the set closing force--moves away from the cover plate.
In this known valve, the annular orifice of the valve seat is formed by a cylindrical plug loosely fitted into the opening formed in the movable part. The drawback of this solution is that the cylindrical plug may move in a lateral direction, thus the width of the annular orifice along the circumference is not constant. A greater force acts on the cover plate, made of elastic synthetic material, along the wider orifice section, which then takes up an oblique position, since it is supported only by a packing element surrounding the gas space that forms the gas spring, the packing element being formed as an elastic "bell". Owing to the oblique position, the actuating liquid flows out through a section of the annular orifice with higher velocity and at higher pressure. The outflowing actuating liquid exposes a certain part of the cover plate and "bell" behind it to intensive erosion effects, wearing it out, as a result of which they lose their closing and sealing capacity, and the pressure limiting valve becomes inoperative.
At the same time the oblique position of the cover plate entails the unfavorable consequence, whereby the valve does not open at the set required pressure value, thus its functioning is no longer reliable even while operative.
A further deficiency of the known pressure limiting valve is that the breakdown of the bell-shaped packing element that supports the cover plate also means the breakdown of the gas spring at the same time, since liquid flows into the gas space, reducing its elasticity and operating capacity partially or completely. This may occur mainly when suddenly high rock pressure acts on the mine prop, causing such a large quantity of liquid to flow out through the orifice of the valve seat within a short time, that it can not pass out fast enough through the outlet opening, therefor it flows through the defective packing element partly into the gas space.